Tofler estimates that 20 million children and adolescents participate in organized sports annually with disruption of the anterior cruciate ligament (ACL) are among the most frequent musculoskeletal injuries affecting physically active men and women. An estimated 200,000 ACL reconstructions (ACLR) are performed annually as the standard of care based on evidence for improved instrumented laxity, the desire to return to sports play, and evidence for a reduction in future knee injuries.9 These ACL injuries have both immediate and long-term implications for an injured person's quality of life, their risk for osteoarthritis, and long-term disability. Although several studies have noted significant long-term disability due to post injury arthritis in 50% of the knees following ACL injury and ACLR, no studies have determined the factors that predict the incidence and timing of signs and symptoms of arthritis, nor the effect that such factors have on patient-related quality-of-life. In addition, no studies have examined the role that various factors, including post-surgical activity and other post-surgical interventions might have on patient-related quality-of-life, osteoarthritis and recurrent injury or surgery following ACLR. We have established the Multicenter Orthopaedic Outcome Network (MOON), which is a prospective cohort study of 2,362 ACLR patients (and with continual enrollment), in which we've collected detailed patient and injury characteristics, as well as the clinical decisions on meniscal and articular cartilage injuries sustained at the time of ACLR. We propose to conduct a two-year follow-up of this established cohort after their ACLR using nine proposed independent risk factors for quality-of-life (AIM 1), symptoms and signs of OA (AIM 2), and recurrent ligament injury and surgery (AIM 3). The proposed independent risk factors include characteristics of the patients (age, gender, BMI, activity level, clinical knee alignment) and their injuries ("pop" heard at time of injury, concurrent meniscal, articular cartilage, and collateral ligament injuries), and treatment decisions made on concurrent meniscus and articular cartilage injuries. These patient-oriented instruments include the, 1) Knee injury and Osteoarthritis Outcome Score (KOOS), 2) the Western Ontario and McMaster Universities Arthritis Index (WOMAC), 3) the Short Form 36 (SF-36), and 4) the International and Knee Documentation Committee (IKDC) questionnaires. For signs of OA a specialized standing radiograph will be obtained. This study will be the largest prospective cohort study of men and women undergoing ACLR that seeks to identify risk factors for three clinically relevant poor outcomes. Results from this study will advance scientific knowledge by improving our ability to manage and counsel patients with regard to their prognosis, guide decision for intraarticular injuries and future patient activity as well as the potential to identify modifiable independent risk factors for future experimental trials.